1. Field of the Invention
The invention relates to capacity optimization in communication channels. More particularly, the invention relates to optimization of Outer Loop Power Control when the channel is capable of discontinuous transmission (DTX).
2. Description of the Related Art
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems. For example, a CDMA system provides increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) xe2x80x9cTIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), (2) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Project 2xe2x80x9d (3GPP2) and embodied in a set of documents including xe2x80x9cC.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d the xe2x80x9cC.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d and the xe2x80x9cC.S0024 cdma2000 High Rate Packet Data Air Interface Specificationxe2x80x9d (the cdma2000 standard), and (4) some other standards.
Wireless telephone systems are capable of carrying both voice and data over a allocated communication channels. Digital wireless telephone systems are particularly suited to carrying data over the allocated communication channels. It is possible for the system to dedicate a channel to a user, via the user""s Mobile Station (MS), in order to accomplish data transmission. A continuously active channel is preferable when the anticipated data transmission is continuous. With a continuously active channel, the user is able to efficiently transmit or receive a continuous data stream over the allocated active channel of the communication system. However, the exploding increase in the number of packet data applications, such as those used when communicating over the Internet, make allocating a continuously active channel to a single user an over allocation of resources. Additionally, because wireless telephone rates are often tied to connection times, a user may not be willing to use a MS to connect to a remote network if a continuous active channel must be dedicated to the connection.
The designers of wireless telephone systems have recognized the desirability of packet data applications over wireless channels. The designers have also recognized that packet data and the associated bursty transmissions may be transmitted over channels that are not continuously active, but rather, allow for discontinuous transmission (DTX).
To maximize channel capacity, a CDMA communication system incorporates power control. Within any cell of a CDMA system all users transmit in the same bandwidth at the same time and each user""s transmission contributes to the interference experienced by all other users. The power control process is used to adjust the transmit power to achieve a minimum desired signal quality at the receiver. The interference contribution experienced by other users is minimized because the transmit power to each user is minimized. Because the interference level is minimized, the number of user""s that can simultaneously communicate over the channel is maximized.
A closed loop control process is used to control transmission power on both the forward and reverse links in a CDMA system. In closed loop control a transmission is made, a measurement of received power or signal quality is made at the receiver, and feedback is provided to the transmitter.
Closed loop power control is used in the reverse link of a CDMA wireless communication system to ensure the reverse link transmit power is accurately controlled. In reverse closed loop power control, a base station (BS) (or base station controller (BSC)) measures the signal level received from each mobile station (MS) and provides feedback to each MS with instructions to adjust the MS transmit power. The closed loop power control loop attempts to adjust each MS transmit power to cause the reverse link transmit signals from all of the MS in the cell to arrive at the minimum level of power required for each MS to achieve a desired Quality of Service (QoS).
The forward link from the base station (BS) to the mobile station (MS) is no less demanding on a power control loop even though all code channels transmitted from the base station take the same paths to the mobile station. The operation of the forward link power control process is similar to the reverse link process. In forward link power control the MS measures the signal level received from the BS and provides feedback to the BS with instructions to adjust the transmitted power of the code channel associated with that MS. The forward link power control process thus affects the power of the particular MS code channel relative to the other code channels.
In the reverse link process, the base station or base station controller measures the received signal-to-interference (Eb/I0) and compares the measured value to an adjustable threshold known as the power control setpoint. When the measured Eb/I0 is above the setpoint, the base station instructs the MS to reduce the reverse link transmit power by a predetermined amount, e.g., 1 dB. When the measured Eb/I0 is below the threshold, the BS sends the MS a command to increase the reverse link transmit power by a fixed amount.
The forward link process may operate in a complementary manner. The MS measures a received signal-to-interference (Eb/Nt) and compares the measured value to an adjustable power control setpoint within the MS used for the forward link signals. The forward link uses the interference measurement Nt rather than the I0 value used in the reverse link. When the measured Eb/Nt is above the setpoint, the MS instructs the BS to reduce the forward link transmit power in the assigned code channel by a predetermined amount, typically fractions of a dB. When the measured Eb/Nt is below the threshold, the MS sends the BS a command to increase the forward link transmit power in the assigned code channel by a fixed amount.
The values of the respective forward link and reverse link power control setpoints, either at the MS or the BS, largely determine the QoS maintained at the receiver. The QoS is often measured as a Frame Erasure Rate (FER), alternatively known as a Frame Error Rate. As expected, increasing the value of the power control setpoint reduces the FER, thereby providing a higher QoS. Reducing the power control setpoint increases the FER. Adjusting the threshold of the power control setpoint occurs in a process known as Outer Loop Power Control (OLPC). In the forward link the process is known as Forward Outer Loop Power Control (FOLPC) and in the reverse link the process is known as Reverse Outer Loop Power Control (ROLPC). The forward link power control setpoint and the reverse link power control setpoint are independently controlled. There may or may not be any correspondence between the setpoint values used in the forward and reverse links because of the differences in signaling schemes and receiver structures used in the forward and reverse links. The similarity in nomenclature refers to the similarity in function, not to a commonality of value or control.
The receiver incorporates a DTX detection algorithm when the channel is DTX capable. Imperfect determinations of DTX and non-DTX frames result in a power control setpoint threshold that is sub-optimal. A setpoint that is too high results in reduced channel capacity. It is desirable during OLPC to adjust the power control setpoint to compensate for inaccurate DTX and non-DTX indications and thus optimize the power control setpoint and the channel capacity.
Novel techniques are disclosed for outer loop power control for communication over a channel employing DTX. Power control setpoint is adjusted to compensate for imperfect signal detection in a communication channel capable of DTX. The power control setpoint is compensated by estimating a number of erroneous detections and determining a compensation value based in part on the estimate. The power control setpoint is compensated for false Erasure detections by adjusting the power control setpoint by a dynamically determined setpoint back off amount upon detection of a Good frame. The amount of the setpoint back off is a function of the measured signal quality of the detected Good frame and a number of Erasure detections received immediately preceding the Good frame indication. The number of Erasure detections is reset when a Good frame is detected.